Wavelength division multiplexing (WDM) is a technique used to transmit multiple channels of data simultaneously over the same optic fiber. At a transmitter end, different data channels are modulated using light having different wavelengths or, colors for each channel. The fiber can simultaneously carry multiple channels in this manner. At a receiving end, these channels are easily separated prior to demodulation using appropriate wavelength filtering techniques.
The need to transmit greater amounts of data over a fiber has led to so-called Dense Wavelength Division Multiplexing (DWDM). DWDM involves packing additional channels into a given bandwidth space. The resultant narrower spacing between adjacent channels carried by a fiber in DWDM systems demands precision wavelength accuracy from the transmitting laser diodes.
Unfortunately, as laser diodes age, they are known to exhibit a wavelength drift of up to 0.15 nm from their set frequency over about a fifteen year period. This period is well within the expected service life of modern laser diodes. Hence, this wavelength drift is unacceptable as a given channel may drift and interfere with adjacent channels causing cross talk. To remedy this situation most laser transmitters use what is commonly referred to in the art as a wavelength locker to measure drift frequency vs. set frequency. This information can be fed back to a controller to adjust various parameters, such as temperature or drive current, of the laser diode to compensate for the effects of aging and keep the diode laser operating at its set frequency. Most laser transmitters with an integrated wavelength locker use either an etalon or thin film filter to measure the laser wavelength variation.
FIGS. 1A and 1B show a type of conventional wavelength locker configuration. A laser 6 produces a laser beam centered about a set frequency or wavelength. The laser 6 emits a light beam from both a front facet 15 and a back facet 13. The actual modulated light carrying the data channel emerges from the front facet 16, which is coupled to an optical fiber (not shown). The beam 12 that emerges from the back facet 13 is used for monitoring purposes since it has the same wavelength as the beam emerging from the front facet 15. The monitored beam 12 passes through a lens 8. A beam splitter 10 splits a monitored beam 12 into two beams. The first beam 14 passes through the splitter 10 and is received by a first detector 16, hereinafter referred to as the power monitor detector 16. The second beam 20 is deflected and passes through a wavelength filter (etalon) 22 after which it is received by a second detector 24, hereinafter referred to as the filter detector 24.
In operation, the detectors 16 and 24, which may be for example, photodiode or optoelectrical detectors, output an electric signal based on the optical input of the received beam. The first detector 16 receives the first beam 14 and outputs a signal that is a function of the monitored beam's 12 power. The second detector 24 receives the second beam 20 and outputs a signal that is a function of both the monitored beam's 12 power as well as its wavelength. Thus, by mathematically operating on these signals as output by the detectors, 16 and 24, the wavelength of the monitored laser beam 12 can be determined and compared to the set frequency to determine any wavelength drift of the laser's 6 output.
The above configuration includes a beam splitter 10 as well as a filter 22 and second detector 24, positioned perpendicular to the optical axis of the monitored beam 12. Thus, this arrangement takes up an undesirably large amount of space in an optical device package.
FIG. 2 shows an alternate wavelength locker configuration that uses a “stacked” arrangement of detectors. As shown, the filter detector 26 and the power monitor detector 27 are stacked one on top of the other with a filter 28 placed in front of the filter detector 26. A collimated beam 29 strikes both of the detectors, 26 and 27 with the lower portion of the beam 29 first passing through the filter 28 prior to striking the filter detector 26. Unfortunately, in this configuration the center portion of the collimated beam 29 where the power of the beam is the highest is not used. Thus, this configuration is not as sensitive to detect small changes in the beam as is desired.
Since optoelectronics packaging is one of the most difficult and costly operations in the manufacturing process, designers are always striving for simpler more compact cost effective arrangements and solutions.